Kimberly Urban at the University of Delaware and Wen-Jun Gao at Drexel University College of Medicine found that any short-term boost in mental performance due to smart drugs may come at a heavy cost: a long-term decrease in brain plasticity, necessary for task switching, planning ahead, and adaptive flexibility in behaviour.

Methylphenidate is a popular smart drug and is often sold on the black market. It was originally developed as a prescription-only drug (sold as Ritalin and Concerta) to treat Attention deficit hyperactivity disorder (ADHD), and works by increasing the level of neurotransmitter in the nervous system.

Trials on rats have shown that young, developing brains are particularly sensitive to methylphenidate: even low dosages early in life can reduce nerve activity, working memory, and the ability to quickly switch between tasks and behaviours.

Another popular smart drug is modafinil, sold under the name Proviigil against narcolepsy and other sleep disorders.

Believed to work by raising levels of dopamine in between synapses of brain nerve cells, it can boost memory as well as the ability to work with numbers and do other mental tasks.

But research indicates that modafinil could have similar long-term undesired effects as methylphenidate on the developing brain, the study authors said.

Not yet widely used are ampakines, an emerging class of drugs currently studied by the US military with the aim of increasing alertness in soldiers.

Ampakines bind to so-called AMPA receptor molecules in the nervous system and boost the response of nerve cells and strengthen connections between them.

Known to improve memory and cognition in rats and healthy humans volunteers, ampakines are often considered to be relatively safe potential smart drugs.

But they are not without dangers for young people: uncontrolled use might over-excite the nervous system, damaging or killing nerve cells, the authors cautioned.

More research on the long-term effects of methylphenidate, modafinil, ampakines, and other smart drugs, especially in young people, is urgently needed, the authors said.

"What's safe for adults is not necessarily safe for kids. The human brain continues to develop until our late twenties or early thirties. Young people are especially prone to abuse smart drugs, but also more vulnerable to any side-effects," Urban said.

The study was published in the journal Frontiers in Systems Neuroscience.